To achieve a low acoustic noise level in motor vehicles, it is known to be beneficial to reduce the vibrations of the panels separating an engine room from a passenger compartment such as a dashboard, and to prevent the transmission of vibrations through the panels. For instance, the rigidity of the panel may be increased by increasing the thickness of the panel, and providing ribs and beads in the panel. To add a damping property to the panel, an asphalt sheet may be thermally bonded to the panel, or viscoelastic material may be sprayed over the panel. This is known to be effective in reducing the vibration transmissivity of the panel.
However, these measures inevitably increase the weight of the vehicle body, and the resulting increase in the thickness of the panel reduces the space available for the passenger compartment. The resulting increase in the mass of the panel lowers the resonance frequency of the panel so that the insulation of low frequency noises may not be achieved in a satisfactory manner.